"You claim to have new dynamite evidence against me. How can this be the case when the document you referred to on the radio is not new; it is a document produced by the regulator last year?

"I have no idea what the case you mentioned has to do with me other than to assume you are using it as a further opportunity to attack me."

The BBC understands this document is not in the public domain.

Sir Philip's letter ends: "You will today receive a letter from my lawyers which you should read very very carefully in order that you clearly understand the content and the seriousness of your actions."

Last week Mr Field gave a Talk Radio interview in which he claimed to have obtained new evidence about the settlement Sir Philip made with the Pensions Regulator over BHS in February.

The BBC understands that Sir Philip's lawyers' letter warned that any disclosure of that evidence would be a breach of pensions law.

Mr Field and Sir Philip have a long history of clashes over the collapse of BHS and subsequent uncertainty over its pensions scheme.

In July 2016, Sir Philip, the former BHS owner, demanded an "immediate apology" from Mr Field after the co-chair of an inquiry into BHS compared Sir Philip unfavourably to late media magnate Robert Maxwell, who took millions from the Mirror Group's pension funds.